1. Field of the Invention
The present invention relates to an AC-DC converter which receives an alternate current power and outputs a direct current voltage, and particularly to a switching power supply apparatus including a Power Factor Correction (“PFC”) converter which improves a power factor.
2. Description of the Related Art
Conventionally, harmonic current restriction is in effect in various categories divided in accordance with usage and input power based on “Japanese Guidelines for Reduction of Harmonic Emission Caused By Electrical and Electronic Equipment for Household and General Use” in Japan, and similarly, the harmonic current is restricted in European countries and various other countries around the world. Therefore, a PFC converter is added to power supply apparatuses for typical home electronics so as to suppress the harmonic current (see, for example, Japanese Unexamined Patent Application Publication No. 5-191976).
An example of a general switching power supply apparatus, which receives a commercial AC power as an input power, rectifies and smoothes the commercial AC power and converts the commercial AC power into a DC voltage, and thereafter, performs switching of the DC voltage using a DC-DC converter. Accordingly, current is discontinuously supplied and is significantly shifted from a sine wave. This causes harmonic current.
In order to suppress the harmonic current, a PFC converter is provided after a full-wave rectifying circuit and before a smoothing circuit including a smoothing capacitor.
The PFC converter includes a chopper circuit. Since the PFC converter operates so that a waveform of an input current and a waveform of an input voltage are in phase, that is, they are approximated to a sine waveform, harmonic current is suppressed to a desired level or less.
The PFC converter is connected to a general isolated DC-DC converter which supplies a voltage to a load circuit.
FIG. 1 is a circuit diagram illustrating a switching power supply apparatus including a conventional PFC converter and an isolated DC-DC converter. The switching power supply apparatus 100 includes a PFC converter 10 which generates a predetermined DC voltage in response to an AC input power Vac input from terminals P11 and P12, and an isolated DC-DC converter 20 which receives the predetermined DC voltage output from the PFC converter 10 and supplies a predetermined DC power voltage to a load circuit 30 through terminals P21 and P22.
The PFC converter 10 includes a diode bridge B1 which performs full-wave rectification on the AC input power Vac, a switching circuit including a switching element Q1 which performs switching of a voltage output from the diode bridge B1, an inductor L1 which allows a current intermittently supplied by the switching element Q1 to flow and which accumulates and discharges exciting energy, a diode D1 which is disposed after the inductor L1 and which rectifies a voltage output from the inductor L1, and a smoothing capacitor C1 which smoothes a rectified output.
The isolated DC-DC converter 20 includes a transformer T1, a switching circuit including a switching element Q2 which is connected to a primary winding N1 included in the transformer T1 and which performs a switching of an output from the PFC converter 10, a diode D2 which rectifies an AC voltage generated in a secondary winding N2 included in the transformer T1 due to turning on or off of the switching element Q2, and a smoothing capacitor C2 which smoothes a rectified output. Furthermore, a control circuit 21 detects an output voltage and applies a switching control pulse to a drive circuit 22 of the switching element Q2 through a photocoupler PC1.
As described above, the control circuit 11 which performs the switching control of the PFC converter 10 is required to detect an input voltage and an input current since the control circuit 11 controls an input current and an output voltage. Therefore, the control circuit is disposed on a primary side of the isolated DC-DC converter 20.
In recent years, a control circuit included in a switching power supply circuit is defined by a digital control circuit, such as a micro computer or a DSP (Digital Signal Processor). An advantage obtained when the digital control circuit is used in the control circuit is that data is transmitted to or received from and signals are input to or output from a control unit included in an electronic apparatus, i.e., a load circuit. For example, a current state of operation of a converter can be transmitted from the digital control circuit included in the converter to the load circuit, and instructions for determining a timing when a voltage is output, a terminal defining a destination to which the voltage is output, and a magnitude of the voltage can be received from the load circuit.
However, to attain this advantage, an isolated unit (such as a pulse transformer or a photocoupler) which transmits data supplied from a secondary side to the digital control circuit while maintaining isolation of a primary side and the secondary side is required. However, a number of pulse transformers or photocouplers corresponding to a number of ports used for data communication are required. Accordingly, this type of digital control circuit is not realistic or feasible in terms of cost and space. Furthermore, when the isolated unit is used for signal transmission, it is difficult to transmit linear signals.